A relatively new combustion strategy known as homogeneous charge compression ignition shows great promise in reducing undesirable emissions from internal combustion engines that utilize a compression ignition strategy. HCCI refers generally to the idea of mixing fuel with air in the engine cylinder before autoignition conditions arise. The mixture is compressed to autoignition, with a general desirability that the combustion event take place in the vicinity of top dead center. Although HCCI has proven the ability to drastically reduce undesirable emissions, especially NOx, the combustion strategy has brought new problems that must be overcome in order to render such an engine commercially viable. For instance, for an HCCI engine to be a viable alternative to a counterpart diesel engine, it must have the ability to operate effectively over a relatively wide load range. One problem encountered with HCCI engines is the extreme sensitivity and difficulty in controlling ignition timing. In addition, HCCI engines can have difficulty in operating in higher load ranges where more fuel is supplied to the individual cylinder. This perceived limitation may be due to extreme pressure spikes that occur when the charge burns. The pressures can get so high as to exceed the structural containment capability of an engine housing. In addition, extreme pressure rise rates can also limit a load range for an HCCI engine due to extreme vibrations. Thus, controlling an HCCI engine, especially at higher speeds and loads, can be extremely problematic, but must be overcome to enable such an engine to be a viable alternative to a counterpart diesel engine.
Apart from those problems identified in the previous paragraph, multi-cylinder HCCI engines have even more problems that need to be overcome. For instance, engine geometry, including the intake geometry, fuel injector performance variations, and other known and unknown influences affect the specific burn behavior of charges in different ones of a plurality of engine cylinders. For instance, intake manifold geometry may result in one cylinder receiving less or more air than other cylinders, and a fuel injector performance variation in another cylinder may cause less or more fuel to be injected based upon the same control signal. These differences result in different air fuel ratios in different cylinders. Homogeneous charge combustion is very sensitive to air fuel ratio and other factors. Thus, these variations can contribute to substantial differences in both energy release and combustion phase timing among a plurality of different engine cylinders. While some variation may be more acceptable at lower load conditions, at higher speeds and loads, combustion phasing and energy release variation among the plurality of cylinders can give rise to unacceptable vibration and noise levels. In addition, the cylinder with the most advanced combustion will develop a maximum pressure at a lower load, and therefore will limit peak load of the entire engine. Those skilled in the art will appreciate that, when the charge burns too early in the combustion stroke, excessive cylinder pressures and pressure rise rates can occur that limit the load carrying capability of that cylinder, and hence the entire engine.
One strategy for dealing with the problems of HCCI is described in co-owned U.S. Pat. No. 6,725,838. This reference describes a mixed mode strategy where HCCI is employed over a lower load range portion of the engine, and conventional diesel engine strategies are employed at high speeds and loads. By conventional, this disclosure means that fuel is injected directly into an engine cylinder after autoignition conditions have arisen, with the injection taking place in the vicinity of top dead center. While a mixed mode strategy can produce superior emissions compared to that of a conventional diesel engine, it requires increased complexity, especially in manufacturing and controlling the fuel system, but mixed mode still results in more undesirable emissions than that possible with an entirely HCCI operation strategy.
The present disclosure is directed toward one or more of the problems set forth above.